In recent years, with reduction of a supply voltage which is used, a threshold voltage of a semiconductor device also tends to be reduced. When the threshold voltage is reduced, malfunction due to noise is likely to occur. From such a situation, a driving circuit devise to reduce occurrence of the above malfunction is desired. As such a driving circuit, a driving circuit including two switches is widely known. In the driving circuit, when a semiconductor device is turned on, one switch is turned on to apply a positive voltage to the gate, and when the semiconductor device is turned off, the other switch is turned on to apply a negative voltage to the gate. In such a structure, a gate voltage at the turn-off is greatly far from a threshold potential. Therefore, even when the noise at switching and the like is superimposed on the gate, malfunction (erroneous on) due to that can be prevented.
However, in the above driving circuit, in addition to a power circuit that generates a positive power supply, a power circuit that generates a negative power supply is needed. Therefore, downsizing of the overall circuit is difficult, and the production cost increases. On the other hand, a driving circuit to generate a negative voltage by its own operation is disclosed in Japanese Patent No. 4682173 (corresponding to US 2009/0033377 A1 and called Patent Document 1). In the driving circuit of Patent Document 1, positive and negative voltages whose absolute values are each equal to a supply voltage can be applied from one driving power supply to the gate of semiconductor device.
However, the driving circuit of Patent Document 1 needs five switches (semiconductor switches). That is, compared with the above driving circuit having two switches, the number of switches greatly increases in the driving circuit of Patent Document 1. A relatively great current flows in the switches that form the driving circuit at a moment of switching. Therefore, as a switch used for the driving circuit, a capability (size) that can withstand such a current is needed. Therefore, increase of the number of switches used for the driving circuit causes problems such as increase of a circuit size and increase in cost. Particularly, since rapid switching needs charging and discharging of the gate in a short time, a great current flows in the driving circuit at an instant of switching, which makes the above problems remarkable.
Additionally, for the actual structure of a driving circuit, a short cut protection is necessary to stop the turn-on when a flow of a continuous short cut current is detected in the driving circuit. As a method of detecting a current (short-circuit current), a method of detecting a current based on a voltage drop of a shunt resistor, a method of detecting a current based on an amount of a voltage drop at a turn-on in a semiconductor switch, etc. are mentioned.
In the former method, for example, a resistor is inserted in series relative to a power supply terminal or a ground (GND) terminal, and a current value is detected based on a voltage drop due to the resistor. In such a method applied in the driving circuit, a current drive capacity at the on and off is restricted by the resistor. On the other hand, in the latter method, a voltage (for example, a voltage between a drain and source) of a semiconductor switch in which a short cut current may flow is monitored (detected), and it is determined that an excess current has flown (short cut) at a remarkable voltage drop also at the turn-on. When such a method is applied to the driving circuit, a current drive capacity at the on and off is not restricted. Therefore, it is effective in the driving circuit to use the latter current detection method.
However, when the short cut protection of the circuit using the above latter current detection method is provided to the driving circuit of Patent Document 1, the following problem occurs. That is, two paths in which an excessive current (short circuit current) may flow from the power supply exist in the driving circuit of Patent Document 1. Therefore, two current detecting circuits to detect voltages of two semiconductor switches that exist in the respective paths are needed. That is, in the driving circuit of Patent Document 1, the short circuit protection of the circuit becomes complicated. Thus, there is the problem that the circuitry is complicated in the driving circuit of Patent Document 1.